Data communication using digital terminals connected to one another via analog telephone lines has become more prevalent, and modems for performing conversion of analog signals and digital signals from one to the other or vice versa between the digital terminals and the analog telephone lines have been used widely. More specifically, as shown in FIG. 1, a modem DCE has functions of receiving data in the form of a digital signal from a terminal DTE, modulating the input data with a predetermined modulation technique, and outputting an analog signal to the line side, while it also has functions of demodulating an analog signal sent from the opposite side via the line with a predetermined demodulation technique and outputting a demodulated digital signal to the terminal connected to the other end thereof. Thus, DTE1 and DTE2 on both the sides can perform data transfer simultaneously or alternately dependent on communication modes. There are various communication modes as follows:
(1) Full duplex . . . capable of inputting and outputting data simultaneously
Half duplex . . . inputting and outputting data alternately (only either one of inputting and outputting is possible at the same time)
(2) Modulation technique
frequency modulation (FSK) PA1 phase modulation (PSK) PA1 quadrature modulation (QAM); PA1 300 bps (bps: number of transferred bits per second) PA1 1200 bps PA1 2400 bps PA1 4800 bps PA1 7200 bps PA1 9600 bps. PA1 synchronous PA1 asynchronous.
(3) Communication speed
(4) Data form
Various modes are created by diverse combinations of the above items (1), (2), (3) and (4). Thus, data transmission has variety in signal formats and transmission formats. To make matching with those different formats, some modems are equipped with various operation modes adaptable to the different formats. Further, as transmission standards, there are specified Recommendations from Consultative Committee of International Telegraph and Telephone (CCITT) V.21, V.22, V22bis, V.23, V.26bis, V.27ter, V.29, etc.
For carrying out digital communication via telephone lines, it is essential to match operation modes of modems installed on both the calling and answering sides of a communication system.
A conventional mode matching method has been practiced by (a) manually setting a modem at the other party in advance, (b) setting the modem during the sequence for connection, or (c) replacing the modem with another one as required. However, such a conventional method requires the intricate process to make setting for matching of the modems after exchanging information related to connection conditions between the calling and answering sides prior to connection of a line, and to replace the modem itself with a new one when the setting cannot be completed. Another disadvantage is in that it is difficult for the conventional method to be adaptable for many kinds of operation modes and, even if possible, troublesome operation of shifting a multiplicity of switches is necessitated.
Another conventional method is known which gives an instruction from a terminal to a modem for changing an operation mode of the modem. In this method, however, because a command mode and a data mode are discriminated from each other, shifting from an asynchronous mode to a synchronous mode may become intricate, and shifting from the command mode to the data mode or from the data mode to the command mode may cause an adverse effect on communication which cannot be disregarded. Another disadvantage of this type modem is in that operation modes could not be matched with each other unless the operation of the modem at the other party is known. Accordingly, there has been a desire for a method capable of matching modem operation modes regardless of the operation mode of the opposite modem.
The present invention is to improve the disadvantages as mentioned above, and to provide a method by which operation modes of modems at the transmit and receive ends of a transmission line is automatically matched upon a control command from a terminal. This eliminates intricacy as encountered in the prior art shifting process.